1. Field of the Invention
This invention relates to a method and apparatus for non-destructively testing the state of insulation or the performance of a variety of electrical machines and equipment by applying a high d.c. voltage.
2. Description of the Prior Art
In order to inspect the insulation state of an electric installation such as power cable, various non-destructive test methods have been developed and performed. The high d.c. voltage method which is one of the non-destructive test methods tests the behavior of the insulation, especially the state of deterioration due to the dry or moist condition or to voids or cracks in an insulator, from the current - time characteristic, the insulation resistance - voltage characteristic, etc.
When a d.c. voltage is applied to an insulator, three components of current flow -- a displacement current (Id) (charging current), an absorption current (Ia) and a leakage current (Ir). The displacement current is a current which charges the capacitance of the insulator when the voltage is applied, and it decays instantly. The absorption current is a current which is caused by the polarization of the insulator, and it decays gradually with time. The leakage current is a conduction current which flows through the interior or surface of the insulator, and it is substantially constant versus time.
The resultant current of the three components is measured as the total current. Since the current value itself varies in dependence on the size, configuration etc. of a specimen, it is difficult to judge the behavior of the insulation simply from the magnitude of the current. When the specimen moistens or deteriorates, the leakage current becomes very large. Therefore, that proportion of the total current which is constituted by the leakage current (which is constant versus time) increases, and the rate of decay of the current decreases. Exploiting this fact, the current - time characteristic employs the time variation of the current as the criterion of the moist or dry state. The polarization index (P. I.) which is defined by Eq. (1) or (2) is used as an indicator.
In terms of the leakage current value, ##EQU1##
In terms of the insulation resistance value, ##EQU2##
The time "n min." differs in dependence on the configuration, material etc. of the specimen to be tested, and is determined to be "3 min.," "5 min.," "7 min.," "10 min.," etc. When the currents (or insulation resistances) after 1 min. and n min. are measured, the applied voltage must have been raised to a prescribed voltage for the test. It is accordingly necessary to complete the charging of the test piece and raise the applied voltage to the prescribed value within at the latest 1 min.
In prior-art test apparatus for evaluating the current - time characteristic, however, as illustrated in FIG. 1, the internal resistance R of the test apparatus is made large to protect the test apparatus from the surge of charging current to the capacitance Ca of the test piece Rx (whose equivalent circuit is shown in the figure) at the initiation of the application of the voltage. Therefore, the applied voltage Vo is gradually raised with a time constant which is determined by the internal resistance R and the capacitance Ca. The current Io fluctuates greatly in dependence on the specifications of the test piece. Moreover, it is impossible to raise the applied voltage to the prescribed value at 1 min. after the application of the voltage. Consequently, it is quite impossible to determine a reliable value for the polarization index.